package main

import (
	"fmt"
	"math"
)

const (
	width, height = 600, 320            //以像素表示画布的大小
	cells         = 100                 //网格单元的个数
	xyrange       = 30.0                //坐标轴范围 -xyrange ~ +xyrange
	xyscale       = width / 2 / xyrange //x 或 y 轴上每个单位长度的像素
	zscale        = height * 0.4        //z 轴上每个单位长度的像素
	angle         = math.Pi / 6         //x 、 y 轴的角度(30°)
)

var sin30, cos30 = math.Sin(angle), math.Cos(angle) // sin30° cos30°

func main() {
	fmt.Printf("<svg xmlns='http://www.w3.org/2000/svg' "+
		"style='stroke: grey; fill: white; strocke-width: 0.7' "+
		"width='%d' height='%d'>", width, height)

	for i := 0; i < cells; i++ {
		for j := 0; j < cells; j++ {
			ax, ay := corner(i+1, j)
			bx, by := corner(i, j)
			cx, cy := corner(i, j+1)
			dx, dy := corner(i+1, j+1)
			fmt.Printf(
				"<polygon points='%g,%g,%g,%g,%g,%g,%g,%g,'/>",
				ax, ay, bx, by, cx, cy, dx, dy)
		}
	}
	fmt.Println("</svg>")
}

func corner(i, j int) (float64, float64) {
	//求出网格单元(i, j)的顶点坐标(x, y)
	x := xyrange * (float64(i)/cells - 0.5)
	y := xyrange * (float64(j)/cells - 0.5)
	//计算曲面高度 z
	z := f(x, y)

	//将(x, y, z)等角投射到二维SVG绘图平面上，坐标是(sx, sy)
	sx := width/2 + (x-y)*cos30*xyscale
	sy := width/2 + (x-y)*cos30*xyscale - z*zscale

	return sx, sy
}

func f(x, y float64) float64 {
	r := math.Hypot(x, y) //到 (0, 0) 的距离 疑似勾股定理
	return math.Sin(r) / r
}
